[Skyli (OP)]

If I understand correctly then every element with neutrons is subject to beta-decay (even if the half-life is longer than the age of the universe) and, during beta-decay, a neutron rips itself apart to form another proton while ejecting electrons or positrons and a photon. In other words, an atom moves "up" the periodic table to the element with the next atomic number (as determined by the number of protons in the nucleus).

What happens to Deuterium? Does it form a Helium atom without neutrons, does it just split into two hydrogen nuclei or is it, like hydrogen, not subject to beta-decay at all?

[Kryptid]

Off the top of my head, it shouldn't decay like that. Decay occurs when the product is more stable (in a lower energy state) than its precursor. Helium-2 (a helium nucleus without neutrons) is highly unstable, so it can't decay into that. A pair of protium (hydrogen-1) nuclei have a higher mass than a deuteron, so it can't decay that way either. My best guess is that it will most likely decay via proton decay (if that occurs).

[CliffordK]

Deuterium is considered stable.

Tritium decays via beta decay to helium-3, with 2 protons and 1 neutron.

[Skyli (OP)]

Thank you guys.

That Tritium decays to He-3 makes sense and current understanding suggests that all the Deuterium in the universe originated shortly after the Big Bang (Stars fuse it faster than they produce it so they cannot be the source). That's pretty stable, to be sure.

However, if both Hydrogen and Deuterium are stable, why is the ratio so different? Most elements have an equal or greater number of neutrons to protons in their natural forms so I would have expected Deuterium to be prevalent.

[chiralSPO]

Could you have supplied the answer to your own question in your post? Perhaps because stars eat deuterium more quickly, there is less. I don't know what rate difference there is.

[alancalverd]

You need an excess of neutrons to undergo beta decay. Deuterium does not have an excess, just more than protium.

[Kryptid]

Or perhaps because in order to have protium, all you need is a lone proton. To make deuterium, a relatively chance encounter is required that will bring a proton and a neutron close enough to each other to be bound together by the strong nuclear force (deuterium produced by radioactive decay notwithstanding, assuming it can be produced in such a way).

[Skyli (OP)]

I must admit, ChiralSPO; your observation made me a bit red in the face for a moment there, great! Although I would expect the stars to be "starving" by now and yet they seem to be forming and living as ever. Interesting thought though; how do they make up for the deficit? If you are right, would that determine the maximum age of the Universe?

The "chance encounter" explanation from Supercryptid makes sense, but are such encounters really so infrequent? Less than 0.02% over the life of the universe seems low, especially if all the Deuterium was made in or shortly after the Big Bang. Everything was encountering everything else at a pretty ferocious rate back then I would expect.

[chiralSPO]

Didn't intend to embarrass you, Skyli. As far as I know, you don't need deuterium to form or sustain a star as long as it is big enough. A large enough star can sustain the CNO cycle to form He from H (which is more efficient than simple H fusion once the star is hot enough).

[Skyli (OP)]

Not to worry chiralSPO, I'm thick-skinned

Thanks for the information on the CNO cycle. I knew it went on in bigger stars but, after a good Wiki, I now know it really is cyclic.

To be absolutely honest, my original query was answered when CliffordK informed me that Deuterium was stable. Now the only mystery left is why there is so little. I'm a bit dubious about the "chance encounter" proposition put forward by Supercryptid for the reasons I've already stated.

Any ideas on why Deuterium is in such short supply?
Anyone?

[Phractality]

Beta decay is something that occurs in the neutron, changing it to a proton and ejecting an electron. It doesn't matter what kind of nucleus the neutron belongs to. Any atom that contains one or more neutrons is subject to beta decay.
 
When beta decay occurs in deuterium, you get a nucleus consisting of two protons and no neutrons. Such a nucleus cannot remain together for any measurable time; the two protons will fly apart instantly unless they happen to be joined  by a passing neutron at the exact instant the neutron decays. So nearly all the time, you get two atoms of protium (H_¹), and those are unlikely to become part of helium nucleus for many millennia.